Current mirror circuits are well known and widely used in various applications, especially for providing controlled currents in integrated circuits. A current mirror circuit makes use of a reference current source to determine a mirrored current. In the absence of an accurate reference current source, it is common to derive the reference current via a resistor coupled to a power supply voltage. When the supply voltage is sufficiently well regulated, this can provide a sufficiently accurate reference current, but in other situations, for example when it is desired to avoid costs and loss of efficiency associated with supply voltage regulation, it becomes desirable to compensate for supply voltage variations in the generation of the reference current for the current mirror circuit.
In addition, there can be circumstances in which it is desirable to control currents generated by current mirror circuits in a particular predetermined manner with variations in supply voltage. For example, for optimum operation of a transmitter power amplifier of a portable cellular radio telephone with varying battery supply voltage, it may be desirable to increase a bias current as the supply voltage falls in order to extend a low-voltage working range of the telephone, and/or it may be desirable to limit this bias current in the event that the supply voltage exceeds a normal working range, thereby providing an over-voltage shutdown function.
By way of example, Prak U.S. Pat. No. 3,875,430 issued Apr. 1, 1975 and entitled "Current Source Biasing Circuit" discloses a current source in which a square law dependency of output current on supply voltage is overcome by operating a field effect transistor in an exponential portion of its current-voltage characteristic, making the output current substantially independent of the supply voltage. Joseph U.S. Pat. No. 4,399,399 issued Aug. 16, 1983 and entitled "Precision Current Source" discloses a current source using a two-transistor current drive network in which a ratio of current densities is controlled to a predetermined value using an amplifier feedback circuit, an output current being produced dependent upon the ratio of the current densities and independent of supply voltage variations. Yamada et al. U.S. Pat. No. 4,591,780 issued May 27, 1986 and entitled "Constant Current Source Device Having A Ratio Metricity Between Supply Voltage And Output Current" discloses a current source in which an output current changes at substantially the same rate as changes in the supply voltage.
Other current source circuits are known which provide compensation for process and/or temperature variations, and/or which use two or more current mirror circuit stages. Examples of these are found in Djenguerian et al. U.S. Pat. No. 5,038,053 issued Aug. 6, 1991 and entitled "Temperature-Compensated Integrated Circuit For Uniform Current Generation"; Ryat U.S. Pat. No. 5,498,952 issued Mar. 12, 1996 and entitled "Precise Current Generator"; Perraud U.S. Pat. No. 5,838,149 issued November 17, 1998 and entitled "Voltage Control Means Having A Reduced Sensitivity To Temperature Variations"; Kothandaraman et al. U.S. Pat. No. 5,847,556 issued Dec. 8, 1998 and entitled "Precision Current Source"; and Brown et al. U.S. Pat. No. 5,864,228 issued Jan. 26, 1999 and entitled "Current Mirror Current Source With Current Shunting Circuit".
An object of this invention is to provide an improved current mirror circuit.